Because the question keeps coming up repeatedly in the forum, I want to repeat the
most important pieces of advice from the last
thread in this blog post. Firstly and most importantly:

If you use SharpDevelop 2.x, you must use TortoiseSVN 1.4.x.
If you use SharpDevelop 3.x, you must use TortoiseSVN 1.5.x.

So why is this important? Well, Subversion 1.5 does change the working copy format,
therefore older clients cannot read the working copy anymore. And why is this important
to SharpDevelop? Daniel continues to explain:

SharpDevelop uses its own copy of the Subversion library for marking files
as added/removed/etc. This can upgrade the working copy to the 1.5 format.

However, SharpDevelop does not have it's own commit dialog - we simply
start TortoiseSVN.

So there are two Subversion libraries used by SharpDevelop:
the one we ship for local operations, and the one shipped with TortoiseSVN for commit
etc. Because the Subversion 1.4 library cannot read working copies touched by the
1.5 library, you run into this kind of problem when either TortoiseSVN or SharpDevelop
is too old.

To sum up: SharpDevelop 2.x ships with libraries that are compatible with Subversion
1.4 and requires TortoiseSVN 1.4 for UI. Whereas version 3.0 of SharpDevelop ships
with (different) libraries that are compatible with Subversion 1.5 and requires TortoiseSVN
1.5 for UI.

So will there be an upgrade for SharpDevelop 2.x to support Subversion 1.5? This was
a hotly debated issue, however, in the end it boiled down to the following question:
do we want to ship two almost identical versions of SharpDevelop 2.2 that only differ
in the version of the included Subversion library (because we would have to support
both camps)? As that would create quite a bit of confusion, we decided to leave 2.2
as is and support Subversion 1.5 only in SharpDevelop 3.0.

Running SharpDevelop on USB thumb drives has been possible for a long time. What is
new in version 3.0 though is that the settings that usually go into the user's profile
can live directly on the memory stick - allowing you to take your settings with you
at all times. Here is the procedure:

Download SharpDevelop 3.0 and perform a standard installation on Windows. By default
the installation location (on an x64 machine) will be the following:

Simply copy the 3.0 directory to your memory stick, and then open the \bin directory
there:

There, you have to open SharpDevelop.exe.config - and search for the appSettings section
which by default looks like this:

<appSettings>
<!-- Use this configuration setting to store settings in a directory
relative to the location
of SharpDevelop.exe instead of the user's profile
directory. -->
<!-- <add key="settingsPath" value="..\Settings" /> -->

<!-- Use this setting to specify a different path for the code completion
cache.
The cache contains information about referenced
assemblies to speed up loading
the information on future SharpDevelop
starts. -->
<!-- <add key="domPersistencePath" value="..\DomCache" /> -->

<!-- Use this setting to disable the code completion cache. Code completion
will still be
available, but take longer to load and use more
RAM. -->
<!-- <add key="domPersistencePath" value="none" /> -->
</appSettings>

The comments tell you what to do, basically uncomment settingsPath and domPersistencePath: